The present invention relates to tuned exhaust systems for two-stroke engines.
Two main goals of a designer of exhaust systems for two-stroke engines are to provide a design capable of ensuring that the fuel-air mixture trapped for combustion in the engine will be optimized resulting in high specific power output while minimizing exhaust noise. Other design goals are to accomplish the main two goals in a design which is compact, relatively inexpensive and versatile.
The prior art includes a variety of attempts to accomplish one or more of the aforementioned goals and examples of those attempts which most resemble applicants' exhaust system are disclosed in U.S. Pat. No. 3,462,947 issued to Nowak on Aug. 26, 1969 and French patent application Ser. No. 75 31425 filed by Bancel et al on Oct. 14, 1975 and published under No. 2,328,110 on May 13, 1977.
The patented Nowak exhaust system is a relatively compact system including an inlet pipe projecting through a first closed end of a cylindrical shell and joined to a conical section which diverges toward and terminates in spaced relationship to a reflector plate positioned against a second closed end of the shell. Disposed in spaced concentric relationship to the conical section is a conical member which converges in a direction away from the reflector plate and cooperates with the conical section to increasingly restrict the flow of gases reflected from the reflector plate. The conical member has its large end fixed in sealing relationship to the cylindrical shell to prevent gases reflected from the reflector plate from flowing directly to an outlet pipe joined to the side of the shell. A pair of annular perforated plates are fixed to the conical member at respective spaced locations between the first closed end of the shell and the outlet pipe to thereby cooperate with the first closed end, each other and the conical member to form three expansion chambers into which the gases flow on the way to the outlet pipe. One drawback associated with the patented Nowak design is that the parts thereof are not suited to be re-arranged for providing an effective and efficient exhaust system for several different sizes of engines nor is the design particularly suited for easy assembly.
The exhaust system shown in the published Bancel et al application comprises a tubular shell including an inlet at one end from which a conical portion thereof diverges, the large end of the latter being joined to a cylindrical portion of the shell. A conical reflector member is concentrically located within the cylindrical shell portion and is mounted for axial adjustment therein with its small end facing in the direction of the inlet of the shell. Thus the reflector member cooperates with the cylindrical portion of the shell to present an increasing restriction to gas flow as it passes from the small to the large end of the conical reflector member. While the Bancel et al exhaust system is relatively simple and the reflector member may be adjusted to tune the system for use with several sizes of engines, the design does not demonstrate sufficient noise damping characteristics.